There is a growing trend to develop smaller and smaller devices by increasing the number of components fabricated on a single wafer. In the field of high-speed, wide-band optical signal processing and interconnecting technology, a small-sized, low-cost, and efficient modulator is needed. One of the most promising modulators for such applications is a semiconductor surface-normal modulator. It has many advantages, such as low power consumption, small size, polarization insensitivity, simplicity in light coupling, and fiber pigtailing. Surface-normal modulators are especially useful in applications where a high density of output is desired.
Mode-locked lasers are compact sources of ultra-short pulses at high repetition rate. Today there is a great need for mode-locked lasers that provide simultaneously an efficient pulse shaping mechanism producing high-quality pulses with sub-picosecond duration and synchronization to an external clock signal. Several surface-normal devices have been demonstrated to provide control over the pulse repetition rate of the mode-locked pulse train by employing an electrically or optically controlled saturable absorber, i.e. modulators.
In the following, two solutions will be examined by way of examples with the aid of FIGS. 1 and 2. According to FIG. 1, the structure of an antiresonant Fabry-Pérot quantum well modulator is considered. FIG. 2 shows as an example a mode-locked laser system where an optically pumped saturable Bragg mirror is used.
In general a major problem with surface-normal modulators is low modulation contrast and low efficiency. This arises from the fact that the interaction length with an optical beam is limited to a few micrometers. To overcome the problem of the short interaction length, asymmetrical Fabry-Pérot structures have been used in surface-normal modulators. This kind of modulator typically has a pair of asymmetric mirrors separated by an active layer. However, the optical bandwidth of such Fabry-Pérot structures reduces dramatically as the reflectivity of the mirror increases. An electrical field is introduced by placing several multiple-quantum-wells (MQW) in the intrinsic region of a p-i-n diode. The operation of such devices is based on the shift of excitonic absorption resonance in multiple-quantum-wells due to an applied electrical field.
In FIG. 1 the modulator 100 has a multilayer structure comprising a dielectric coating (TiO2/SiO2) 102, 16-period λ/4 n-GaAs/AlAs layers 105, a p-doped layer (p-GaAs) 103, intrinsic semiconductor material layers (50-period InGaAs/GaAsP), i.e. multiple-quantum-well (MQW) layers 104, an n-doped semiconductor layer (n+-GaAs) 106, and contact layers (Au) 101 and 107. Should a light signal 108 be applied to the modulator, the DBR (Distributed Bragg Reflector) 105 reflects the light backward. When an electrical field is applied to the modulator, it modulates the absorption of MQWs and thus the light signal. However, the electrical field is unevenly distributed in this structure because the contact 101 through which the electric signal 109 is applied is located asymmetrically, to allow the light 108 to be launched in the modulator (i.e. the light cannot penetrate through the metal contact layer). As is apparent from the above, a drawback of this modulator is low modulation contrast.
FIG. 2 illustrates a mode-locked laser system using an optically pumped saturable Bragg mirror. In this example an optical signal is modulated using a control beam generated by an external laser.
A semiconductor laser 204 generates a light signal which is modulated by a modulator 201. Then the modulated light signal is focused by a lens 202 on a saturable absorber 203. The saturable absorber is a passive device in which the control signal from the semiconductor laser 204 and the mode-locked light of the fiber laser 200 overlap spatially. At the other end the linear cavity has a 99% reflective broadband dielectric output coupler 205. The multiple-quantum-well region of the saturable absorber is excited optically by the control light from a laser 206 providing the non-linear absorption changes, such as absorption bleaching or saturation. The changes in reflection modulate the losses within the laser cavity and allow the mode-locked pulse train to be synchronized to a clock signal applied to the modulator 201. A drawback of this construction is that it employs a critical alignment of control and signal beams. Another drawback of this setup is that it uses a number of expensive components (e.g. control laser, external modulator, lens) which result in high overall cost.
The objective of the present invention is to overcome the problems described above by providing a modulator that is efficient, small in size, easy to construct, and is economically advantageous to manufacture.